264 Evolution and Distribution of Fishes 



of these were continued into Lower Old Red or Devonian 

 times, while other and more highly modified but ungainly 

 types reached a climax of cuirassed perfection, to become 

 extinct toward the close of the Upper Devonian period. 

 Meanwhile types like Birkenia and Lasanius, of late 

 Silurian and Devonian times, most nearly led up to incipient 

 elasmobranchs, and all of these inhabited a set of lakes 

 and rivers that were at times connected or at times separate. 

 So the same species, or at least the same genera, are often 

 continuous over much of the above area. 



As already noted (p. 112), widespread volcanic activity 

 during late Silurian, and specially during late Devonian 

 times, probably accounts in great part, for the obliteration 

 of most of these forms, which had largely evolved into 

 awkward heavily mailed mud-bottom feeders. 



In passing now to the Microplacoda or Elasmo- 

 branchii, we would accept it that from some type allied to 

 Birkenia and Lasanius, a series of organisms arose in late 

 Silurian or early Old Red times, some of which gradually 

 evolved and then culminated in the Cladoselachii, the Acan- 

 thodii, and the Pleuracanthidae. For the heterocercal tail, 

 the apparently distinct branchial slits, the rounded snout, 

 the subcephalic mouth, the varied nature and position of the 

 scales or scutes, and the fusiform body in the two above- 

 named genera, all strongly suggest probable evolution from 

 them or from some early related types, of the three groups 

 above named. Moreover, the lithe aspect of the entire 

 animal, as contrasted with the cumbrous and heavy aspect 

 of most of the previous groups, would favor escape and 

 survival amid untoward conditions that caused the unwieldy 

 to succumb. 



But the three groups, Cladoselachii, Acanthodii, and 

 Pleuracanthidae, illustrate well the extreme difficulty of 

 dealing with these early sharks, on account of their soft 

 structure, which seems always to have left only scattered 

 teeth, fin-spines, or detached — rarely shagreen-connected — 

 scales. So temporary generic names — that formerly, and 

 even still in not a few cases, were required, till more accu- 

 rate knowledge was obtained — have been multiplied to a 

 degree that produces confusing synonymy. But it may safely 



